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The integration of additive manufacturing (AM) in design and engineering has prompted a wide spectrum of research efforts, involving topologically optimized solid/lattice structures, multimaterial structures, bioinspired organic structures, and multiscale structures, to name a few. However, except for obvious cases, very little attention has been given to the design and printing of more complex three-dimensional (3D) hollow structures or folded/creased structures. One of the main reasons is that such complex open or closed 3D cavities and regular/freeform folds generally lead to printing difficulties from support-structure-related issues. To address this barrier, this paper aims to investigate four-dimensional (4D) printing as well as origami-based design as an original research direction to design and build 3D support-free hollow structures. This work consists of describing the rough 3D hollow structures in terms of two-dimensional (2D) printed origami precursor layouts without any support structure. Such origami-based definitions are then embodied with folding functions that can be actuated and fulfilled by 3D printed smart materials. The desired 3D shape is then built once an external stimulus is applied to the active materials, therefore ensuring the transformation of the 2D origami layout to 3D structures. To demonstrate the relevance of the proposal, some illustrative cases are introduced.

Soft materials exhibiting unique nonlinear mechanical behavior with large deformation are ubiquitous in nature. In addition, there is a type of soft active materials (SAMs) that can undergo large mechanical deformation. In recent years, soft materials have been developed for impactful applications in aerospace, biomedical devices, and soft robots. However, the traditional manufacturing approaches greatly limit the functionality and performance of structures and devices made of soft materials. 3D printing as an emerging advanced manufacturing technology is considered as an ideal tool to manufacture soft materials. Compared with other 3D printing technologies, digital light processing (DLP)-based 3D printing fabricates 3D objects by localized photopolymerization, and is capable of fabricating 3D objects with highly complex geometries and high resolution. Recently, DLP-based 3D printing has demonstrated its excellent compatibility with a variety of soft materials, and is capable of manufacturing these soft materials into 3D structures and devices with complex geometries, high resolution and unique performance. This reviewer article presents a brief but timely introduction to the most recent advances in DLP-based 3D printing of soft materials.